In a known method of manufacturing a thin film transistor, TFT, and in particular an amorphous silicon TFT of the so-called inverted staggered type suitable for use as a switching element in a display device, layers of insulative material (silicon nitride), undoped amorphous silicon and n-type amorphous silicon are consecutively grown on a substrate provided with a defined metallic layer constituting a gate. Subsequently one or more further metallic layers, for example of chromium and/or aluminium, are deposited over this multi-layer structure and defined by selective etching, using a mask, to form the source and drain contacts. At the same time a portion of the n-type amorphous silicon layer extending between the source and drain contacts and above the channel region is etched away before a final passivation layer is deposited over the contacts and the exposed surface of the intrinsic amorphous silicon layer so as to contact directly with the intrinsic layer over the channel region.
In another known method of producing an inverted staggered type of thin film transistor intended for similar purposes, overlying layers of insulative material (silicon nitride) and undoped amorphous silicon and another insulative layer of silicon nitride are consecutively deposited on a substrate carrying the defined gate. Portions of the upper nitride layer to either side of the eventual channel region are removed by selective etching to expose underlying areas of the intrinsic silicon layer and define the desired contact regions prior to deposition and selective etching of an n-type amorphous silicon layer and metallisation (chromium and aluminium layers) for the source and drain contacts. A portion of the upper nitride layer remains over the channel region. A passivation layer is subsequently deposited over all exposed surfaces.
In a typical active matrix addressed display device, for example a liquid crystal display device suitable for TV display purposes, having one thin film transistor for each picture element, many thousands of such transistors are fabricated together on a single substrate with the appropriate address conductors and picture element electrodes. While the yields obtained heretofore have generally been considered reasonable, it is recognised that there is a need to improve further yield levels to achieve low cost, mass production of these display devices. A major factor in yield levels is the ability to fabricate TFTs having the necessary uniform operational characteristics in the large numbers required. Just one defective thin film transistor or a few transistors exhibiting slightly differing operational characteristics can render the display device unacceptable.